A chemical amplification resist composition is a pattern forming material of forming a pattern on a substrate by producing an acid in the exposed area upon irradiation with an actinic ray or radiation such as far ultraviolet light and through a reaction using the acid as a catalyst, changing the developer solubility of the area irradiated with an actinic ray or radiation and that of the non-irradiated area.
A so-called immersion method of filling a high refractive-index liquid (hereinafter sometimes referred to as an “immersion liquid”) between a projection lens and a sample has been conventionally known as a technique for enhancing the resolution in an optical microscope.
As for the “effect of immersion”, assuming that NA0=sin θ, the resolution and the depth of focus in immersion can be expressed by the following formulae:(Resolution)=k1·(λ0/n)/NA0 (Depth of focus)=±k2·(λ0/n)/NA02 wherein λ0 is the wavelength of exposure light in air, n is the refractive index of the immersion liquid based on air, and θ is the convergence half-angle of beam.
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, when the projection optical system has the same NA, the depth of focus can be made n times larger by the immersion. This is effective for all pattern profiles and furthermore, can be combined with the super-resolution technology under study at present, such as phase-shift method and modified illumination method.
Examples of the apparatus where the effect above is applied to the transfer of a fine image pattern of a semiconductor device is described in Patent Document 1 (JP-A-57-153433 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and Patent Document 2.
Recent technical progress of the immersion exposure is reported, for example, in Non-Patent Document 1 and Patent Document 3. In the case of using an ArF excimer laser as a light source, pure water (refractive index at 193 nm: 1.44) is considered to be most promising as the immersion liquid in view of safety in handling as well as transmittance and refractive index at 193 nm. In the case of using an F2 excimer laser as a light source, a fluorine-containing solution is being studied from the aspect of balance between transmittance and refractive index at 157 nm, but a sufficient solution in terms of environmental safety and refractive index has not yet been found. Considering the degree of immersion effect and the perfection of resist, the immersion exposure technique is expected to be most soon mounted on an ArF exposure machine.
Also, it is pointed out that when the chemical amplification resist is applied to immersion exposure, the resist layer comes into contact with the immersion liquid at the exposure, as a result, the resist layer deteriorates or a component adversely affecting the immersion liquid bleeds out from the resist layer. Patent Document 4 describes a case where when a resist for ArF exposure is dipped in water before and after exposure, the resist performance is changed, and this is indicated as a problem in the immersion exposure.
As for the medium filled between a projection lens and a semiconductor substrate, which is used in the immersion exposure, as described above, water having a refractive index of 1.44 is employed in view of easy availability and safety and by using an exposure machine having a projection lens with NA of 1.2 to 1.35, pattern formation of a semiconductor device in a design dimension up to the 45 nm generation is considered to be possible.
The generation next to the design dimension of 45 nm is 32 nm, and it is considered that NA of 1.65 is necessary for the pattern formation of a 32 nm-generation semiconductor device and in this case, the medium filled between a projection lens and a semiconductor substrate must have a refractive index of 1.8 or more.
Meanwhile, the material of a projection lens having NA of 1.65 is required to have a refractive index of 1.9 or more, and LuAg is currently supposed to be a promising candidate therefor, but its problem of absorbing a large amount of the passing light has not yet been solved.
Furthermore, a candidate medium having a refractive index of 1.8 or more has also not yet been found.
For these reasons, attention is directed toward a method where a special pattern forming method using an exposure machine with a projection lens having NA of 1.2 to 1.35 is used for the pattern formation of a 32 nm-generation semiconductor device.
Several methods have been proposed for this special pattern forming method, and one of these methods is a double exposure process.
The double exposure process is, as described in Patent Document 5, a process of exposing the same photoresist film two times, and this is a method where the pattern in the exposure field is divided into two pattern groups and the exposure is preformed in twice for respective pattern groups divided.
Patent Document 5 indicates that this method inevitably requires a property like a two-photon absorption resist, that is, a property of the photosensitivity or developer solubility being changed in proportion to the square of exposure intensity, but a resist having such a property has not yet been developed.
On the other hand, with miniaturization of a semiconductor, the diffusibility of an acid produced in the exposed area need to be deadly reduced. In Non-Patent Document 2, a compound obtained by fixing an acid to a polymer is reported, but many insufficient points still remain, and improvement of pattern profile, LER, pattern collapse, development defect and the like is demanded.    Patent Document 1: JP-A-57-153433    Patent Document 2: JP-A-7-220990    Patent Document 3: International Publication No. 04/077158, pamphlet    Patent Document 4: International Publication No. 04/068242, pamphlet    Patent Document 5: JP-A-2002-75857    Non-Patent Document 1: Proc. SPIE, Vol. 4688, page 11 (2002)    Non-Patent Document 2: Macromol Rapid Commun., 27, 1590-1595 (2006)